A conductor that connects between two points in a circuit has an electrical resistance R determined by the conductor material and dimensions. The conductor carries a current I proportional to the difference V in voltage between such points, according to Ohm's Law: V=IR. When an electrical load in a circuit is supplied with current from a supply voltage through a conductor, the voltage at the load is equal to the supply voltage level less a voltage drop equal to the product of the resistance of the conductor times the current passing through the conductor. Depending on the operational state of the circuit, loads may draw a variable amount of current at any given time. The operational state of the circuit varies with the conductive state of switching elements and with the input stimulus and output voltages and currents, whether reactive elements are charging or discharging, etc.
Circuit loads are coupled across a potential difference by two conductors, i.e., between different voltages. An IR voltage drop occurs along a conductor carrying current from a VDD supply voltage (or other reference) to the more positive terminal of the load, leading to a reduced supply voltage at that terminal. IR voltage drop considerations are important during integrated circuit design and planning stages. Alternative circuit layouts can be considered in view of the expected IR voltage drop conditions and the requirements of the load devices. IR drops are calculated based on the resistivity of the conductor material, the cross sectional dimensions of the conductor, the length of the conductor between defined points, typical operational states of the circuit such as the number of switching elements toggling simultaneously, and the expected current loading. If it appears that a load device may be adversely affected by IR voltage drop conditions, the load device may be moved to a location closer to a more regulated source along the power supply rails, i.e., to reduce conductor length, or the conductors can be made wider or thicker, in either case reducing their resistance.
Accounting for the IR drops associated with loads can be a complicated matter due, for example, to the complexity and variable shapes of conductive paths. Some programmed layout planning systems have a function for estimating IR voltage drop. A designer might choose power rail conductors that are more than sufficient in number and size, but this uses scarce circuit area. Electromigration or EM hotspots in a power grid is also a consideration.